The Healthy Eating Index and oral health among adults: a cross-sectional study from an Iranian RaNCD cohort data

This study was conducted to investigate the association between decayed, missing, and filled teeth (DMFT) index and nutritional status measured by Healthy Eating Index 2015 (HEI-2015), in Iranian adults. In this cross-sectional study, data from the Ravansar non-communicable diseases cohort study were analyzed. DMFT index was employed as a measurement of oral health. The HEI-2015 score was calculated based on data obtained from Food Frequency Questionnaire and categorized into quartiles. Linear regression models were used to assess the association between HEI-2015 and DMFT. From total of 7549 participants with the mean age of 45.65 ± 7.70, 3741 of them were female (49.56%). The mean of DMFT in the highest quartile of HEI-2015 was lower than the lowest quartile (12.64 ± 7.04 vs. 14.29 ± 7.54, P < 0.001). The mean of DMFT in subject who had higher socioeconomic status (SES (was significantly lower than those with low SES (P < 0.001). The mean of DMFT in the lowest quartile of HEI-2015 was significantly lower than in the highest quartile, after adjusting for confounding variables (ß = − 0.11, 95% CI − 0.54, − 0.30). The increasing dairy intake (β = − 0.08, 95% CI − 0.13, − 0.03) was associated with decreasing DMFT score and increasing refined grains (β = 0.20, 95% CI 0.02, 0.35) and sodium (β = 0.07, 95% CI 0.02, 0.12) intake was significantly associated with increasing DMFT score. A healthy diet was associated with a decrease in DMFT score in the studied population. Following a healthy diet is recommended for oral health.


Methods
Study design and participants. This cross-sectional study was conducted in 2021 using data from the baseline phase of the Ravansar non-communicable disease (RaNCD) cohort study in Ravansar, Kermanshah province, western Iran. The RaNCD study is part of a Prospective Epidemiological Research Studies in Iran (PERSIAN), which has been started by enrolling 10,047 adults aged 35-65 since 2014. Ravansar is a district with both urban and rural areas, located in Kermanshah province in the west of Iran and holds a population of about 50,000. The detailed methodology and design of the RaNCD study has been published in 2019 15 . Participants included all subjects from the baseline phase of the RaNCD study (n = 10,047). Participants with dentures (n = 2457) and missing data (n = 41) were excluded. Finally, 7549 subjects were examined.
Data collection. Using a validated questionnaire, all required information was collected by well-trained personnel of the cohort center through face-to-face interviews. Demographic information, including age, sex, marital status, socio-economic status (SES), and smoking, was recorded online in an electronic data collection form. The standard Persian cohort questionnaire was used to assess the level of physical activity. This questionnaire has 22 questions about sports, work, and leisure-related activities on an average weekday and has been completed as a self-report.
DMFT score measurements. The DMFT index was employed to measure oral health in this study. The DMFT score measured as the total number of permanent teeth that were decayed (D), missing (M), and filled (F).
Healthy Eating Index 2015. Nutritional information extracted from the Food Frequency Questionnaire (FFQ) was applied to calculate the HEI-2015 scores 16 . The HEI-2015 was calculated based on the method described by Krebs-Smith et al. 14 HEI-2015, which encompasses 13 food items. Nine of these 13 items are emphasized to be consumed in adequate quantities which include whole fruits, total fruits, total protein foods, seafood and plant proteins, greens and beans, total vegetables, whole grains, dairy products, and fatty acids. Therefore, participants with the highest intake were given the highest point. The refined grains, sodium, added sugars and saturated fats should be consumed in moderation, and participants with the lowest intake were given the highest point. Accordingly, all participants received a score for each food item, a point from 0 to 10. Finally, the score of all items is added together and the final score is calculated as a number from 0 and 100 (Table 1).
Statistical analysis. Data were analyzed using Stata software, version 14.2 (Stata Corp, College Station, TX, USA). Baseline characteristics of participants across quartiles of the HEI-2015 and DMFT score were reported as mean ± standard deviation for continuous variables and as percentages for qualitative variables. To compare differences across HEI-2015 quartiles and DMFT, we used the one-way ANOVA and Chi square test. Linear regression models were applied to determine associations between HEI-2015 and DMFT score. All statistical analyzes were considered significant according to P-value of < 0.05 with 95% confidence intervals (CIs).

Results
A total of 7549 participants, with a mean age of 45.65 ± 7.70 years, were enrolled. Compared with those in the lowest quartile, participants in the highest quartile of HEI-2015 were younger (P < 0.001). Overall, 3808 (50.44%) were male, 6851 (90.75%) were married, and 858 (11.47%) were current smokers. Participants with the highest HEI-2015 had fewer current smokers than the first quartile (Q1 = 40.44% vs. Q4 = 11.42%, P < 0.001). While the average DMFT was 13.33 ± 7.28, the score was lower among those with higher HEI-2015(P < 0.001) ( Table 2). Table 3 presents the status of decayed, missing, filled teeth based on the baseline characteristics of the participants. The mean number of filled teeth was higher in women (P = 0.015). The mean number of decayed and missing teeth, as well as DMFT in participants with higher SES was significantly more than those with lower SES (P value for all < 0.001). The oral health measured by decayed, missing, filled, and DMFT was better in participants who flossed than in participants who did not (P value for all < 0.001). The mean of DMFT was 11.33 ± 5.95 in participants who brushed once or twice daily, and 17.05 ± 8.17 in participants who never brushed (P < 0.001).
The association between the HEI-2015 and oral health assessed by linear regression model is shown in Table 4. Compared to quartile 1, the mean number of missing teeth in people in quartile 3 and 4 of HEI-2015 index was lower; (ß = − 2.21 (95% CI − 2.06, − 1.81) and ß = − 2.86 (95% CI − 3.27, − 2.45)), respectively. This association remained significant after adjusting for confounding variables including sex, age, SES, education level, and smoking. After adjusting for confounding variables, the mean of DMFT in the highest quartile of HEI-2015 was lower than in the lowest quartile (ß = − 0.11 (95% CI − 0.54, − 0.30)).
Association between the HEI-2015 components and DMFT by linear regression model is presented in Table 5. After adjusting for confounding variables, increasing dairy intake was associated with decreasing DMFT score. In addition, increasing refined grains and sodium intake was associated with increasing DMFT score.

Discussion
To the best of our knowledge, this is the first study that presents the oral health status, and examines the association between the Healthy Eating Index and oral health in a large sample of Kurdish adults. The major finding of the present study was that a healthy diet was significantly associated with a decrease in DMFT score. According to the results of this study, the mean of DMFT among people in the highest quartile of the HEI-2015 score was lower than in the lowest quartile (ß =− 0.11 (95% CI − 0.54, − 0.30)). In addition, after adjusting for confounding variables, increasing dairy and decreasing refined grains and sodium intake were associated with a decrease in DMFT score.
A study in American adults has shown, greater compliance with the DGA is associated with lower odds of untreated caries. The average coronal DMFT decreased as HEI-2015 scores increased, but trends were not consistent in different ethnic or racial groups. American adults who followed the HEI-2015 recommendations were less likely to have untreated coronal caries than those who did not follow the recommendations 17 .
Bawadi et al. have reported that poor diet was significantly associated with an increased risk of periodontal disease in Jordanian adults 10 . A cross-sectional study conducted by Al-Zahrani et al. on 12,110 individuals showed that individuals who maintained a healthy diet were less likely to have periodontitis compared with people who did not 18 . In our sample population, despite being provided with information about healthy nutrition in Table 3. Baseline characteristics according to the condition of teeth. *P-value was obtained one-way ANOVA and Chi square tests. www.nature.com/scientificreports/ all district health centers, most of the people do not follow the guidelines of a healthy diet. Thus, the consumption of fruits and vegetables is less than the recommended and the consumption of salt, sugar and fats is higher than the allowed limit. It is noteworthy that the increase in food prices in recent years in Iran, may contribute to the lower consumption of some of the food groups such as fresh fruits and vegetables, nuts and proteins. According to the findings of the present study, after adjusting for confounding variables, increasing refined grains and sodium intake was significantly associated with increasing DMFT score. Moreover, increasing dairy intake was significantly associated with decreasing DMFT score. Studies in Denmark and India have shown that dental plaque is lower in people who receive dairy products as recommended 19,20 . A prospective study in American adults also investigated the effect of dietary pattern on dental caries and found that a diet based on consuming more sugar and less dairy products increased the risk of dental caries 21 .
In the present study, it was found that mean DMFT score was higher in older people, men, participants with lower SES, and smokers. This difference in age groups may be due to mechanical changes in tooth decay due to aging, including changes in calcium absorption and cariogenic microbiota 21,26 . Similar to these findings, the study of Najafi et al. 22 has shown the effect of socioeconomic inequality in dental caries in 17 provinces of Iran. A systematic review and meta-analysis study (2019) reported an increased risk of dental caries with increased tobacco smoking 23 . In addition, SES and current smoking were also related to HEI-2015. In this study, it was observed that people with higher SES are in higher quartiles of HEI-2015. However, the role of SES in people's  www.nature.com/scientificreports/ food choices and purchasing power is undeniable, and other studies have proven this association 24,25 . Therefore, these factors were adjusted as confounding variables in examining the association between HEI-2015 and DMFT. The association we found between oral health and HEI was independent of these confounding factors.
The results of this study showed that there is a need for more training in the region to increase compliance to a healthy diet. Regular dental examinations and trainings related to oral health also need to be strengthened in Kurdish population.
Study strengths and limitations. One of the limitations of this study was its cross-sectional nature and therefore, causal associations cannot be established based on these findings. We were not able to measure or modulate the effect of genetic factors. A large sample size is one of the strengths of this study. We were able to control most potentially confounding variables.

Conclusion
The finding of the present study showed that a healthy diet was significantly associated with a decrease in DMFT score. According to the results of this study, the mean of DMFT among people in the highest quartile of the HEI-2015 score was lower than in the lowest quartile.
In addition, after adjusting for confounding variables, increasing dairy intake was significantly associated with decreasing DMFT score and increasing refined grains and sodium intake was significantly associated with increasing DMFT score.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.